demo: Expand testing coverage for 16-bit deltas in v1 codec

We had limited coverage for 16-bit deltas previously: V1Custom test only
exercised delta decoding on zero deltas, and VertexDeltas test only had
positive deltas which could hide an implementation error with sign bit
handling.

To fix this, expand decodeVertexDeltas test to have deltas flip the sign
in the second half of the data, and add decodeVertexV1Deltas test which
has the same data encoded statically.

Author: zeux

demo/tests.cpp

@@ -477,6 +477,27 @@ static void decodeVertexV1Custom()
 	assert(memcmp(decoded, kVertexBuffer, sizeof(kVertexBuffer)) == 0);
 }
 
+static void decodeVertexV1Deltas()
+{
+	const unsigned short expected[] = {
+	    248, 248, 240, 240, 249, 250, 243, 244, 250, 252, 246, 248, 251, 254, 249, 252,
+	    252, 256, 252, 256, 253, 258, 255, 260, 254, 260, 258, 264, 255, 262, 261, 268,
+	    256, 264, 264, 272, 257, 262, 267, 268, 258, 260, 270, 264, 259, 258, 273, 260,
+	    260, 256, 276, 256, 261, 254, 279, 252, 262, 252, 282, 248, 263, 250, 285, 244, // clang-format :-/
+	};
+
+	const unsigned char input[] = {
+	    0xa1, 0x99, 0x99, 0x01, 0x2a, 0xaa, 0xaa, 0xaa, 0x02, 0x04, 0x44, 0x44, 0x44, 0x43, 0x33, 0x33,
+	    0x33, 0x02, 0x06, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x66, 0x02, 0x08, 0x88, 0x88, 0x88, 0x87,
+	    0x77, 0x77, 0x77, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+	    0x00, 0xf8, 0x00, 0xf8, 0x00, 0xf0, 0x00, 0xf0, 0x00, 0x01, 0x01, // clang-format :-/
+	};
+
+	unsigned short decoded[sizeof(expected) / sizeof(expected[0])];
+	assert(meshopt_decodeVertexBuffer(decoded, 16, 8, input, sizeof(input)) == 0);
+	assert(memcmp(decoded, expected, sizeof(expected)) == 0);
+}
+
 static void encodeVertexMemorySafe()
 {
 	const size_t vertex_count = sizeof(kVertexBuffer) / sizeof(kVertexBuffer[0]);
@@ -610,9 +631,9 @@ static void decodeVertexDeltas()
 	for (size_t i = 0; i < 16; ++i)
 	{
 		data[i * 4 + 0] = (unsigned short)(0xf8 + i * 1);
-		data[i * 4 + 1] = (unsigned short)(0xf8 + i * 2);
+		data[i * 4 + 1] = (unsigned short)(0xf8 + (i < 8 ? i : 16 - i) * 2);
 		data[i * 4 + 2] = (unsigned short)(0xf0 + i * 3);
-		data[i * 4 + 3] = (unsigned short)(0xf0 + i * 4);
+		data[i * 4 + 3] = (unsigned short)(0xf0 + (i < 8 ? i : 16 - i) * 4);
 	}
 
 	std::vector<unsigned char> buffer(meshopt_encodeVertexBufferBound(16, 8));
@@ -2733,6 +2754,7 @@ void runTests()
 	decodeVertexV0Mode2();
 	decodeVertexV1();
 	decodeVertexV1Custom();
+	decodeVertexV1Deltas();
 
 	for (int version = 0; version <= 1; ++version)
 	{